The present invention relates to an image forming apparatus and more particularly to a developing device therefore capable of increasing image density and improving the quality of a low contrast image.
It is a common practice with a copier, printer, facsimile apparatus or similar electrophotographic or electrostatic image forming apparatus to electrostatically form a latent image on an image carrier in accordance with image data. The image carrier may be implemented by a photoconductive element or a photoconductive belt. A developing device develops the latent image with toner and thereby produces a corresponding toner image. A current trend in the imaging art is toward a magnet brush type developing system using a toner and carrier mixture or two-ingredient type developer. This type of developing system is desirable from the standpoint of image transfer, halftone reproducibility, and stability of development against varying temperature and humidity. Specifically, a developing device using this type of system causes the developer to rise in the form of a brush chain on a developer carrier, so that toner contained in the developer is transferred to a latent image formed on the image carrier at a developing region. The developing region refers to a range over which a magnet brush rises on a developer carrier and contents the image carrier.
The developer carrier is generally made up of a hollow cylindrical sleeve or developing sleeve and a magnet roller surrounded by the sleeve. The magnet roller forms a magnetic field for causing the developer deposited on the sleeve to rise in the form of a head. When the developer rises on the sleeve, carrier particles contained therein rise along magnetic lines of force generated by the magnet roller. Charged toner particles are deposited on each of such carrier particles. The magnet roller has a plurality of magnetic poles formed by rod-line magnetic and including a main magnetic pole for causing the developer to rise in the developing region.
In the above configuration, when at least one of the sleeve and magnet roller moves, it conveys the developer forming a head thereon. The developer brought to the developing region rises in the form of a brush chain along the magnetic lines of force generated by the main magnetic pole. The brush chain or head contacts the surface of the image carrier while yielding itself. While the brush chain or head sequentially rubs itself against a latent image formed on the image carrier on the basis of a difference in linear velocity between the developer carrier and the sleeve, the toner is transferred from the developer carrier to the image carrier.
The problem with the above developing device is that conditions for increasing image density and conditions for desirably developing a low contrast are contrary to each other. It is therefore difficult to improve both of a high density portion and a low density portion at a time. Specifically, the conditions for increasing image density are, e.g., that a gap for development between the image carrier and the sleeve be small and that the developing region be broad. Conversely, the conditions for desirably developing a low contrast image are, e.g., that the above gap be great and that the above developing region be narrow. Therefore, implementing high image quality by satisfying both of the above two different conditions is difficult to practice, as generally accepted.
Particularly, when importance is attached to a low contrast image, it is likely that the crossing portion of solid line images or the trailing edge of a solid black or halftone image is jagged or locally lost. It is also likely that a lattice image formed with the same width has horizontal lines developed thinner than vertical lines or that a solitary dot or similar small dot image is not developed at all.
Technologies relating to the present invention are disclosed in, e.g., Japanese Patent Laid-Open Publication Nos. 5-303284, 6-110333, 6-149058 and 9-43992, Japanese Publication No. 7-117791, and Japanese Patent No. 2,881,823.
It is therefore an object of the present invention to provide an image forming apparatus capable o obviating the jaggedness or local omission of the trailing edge of an image, particularly a low contrast image, for thereby insuring desirable image density and image quality, and a developing device therefor.
In accordance with the present invention, in a method of developing a latent image electrostatically formed on an image carrier by causing a developer to deposit on a developing sleeve in the form of a magnet brush and contact the image carrier, the magnet brush uniformly rises in the form of a head in the axial direction of the developing sleeve and contacts the image carrier.
Also, in accordance with the present invention, in a developing device for developing a latent image electrostatically formed on an image carrier by causing a developer to deposit on a developing sleeve in the form of a magnet brush and contact the image carrier, the magnet brush uniformly rises in the form of a head in the axial direction of the developing sleeve and contacts the image carrier.
Further, in accordance with the present invention, in a magnet roller constituting a developer carrier mounted on a developing device, an auxiliary magnet helps the main magnetic pole form a magnetic force for causing a developer to rise in the form of a head in a preselected developing region.
Moreover, in accordance with the present invention, in an image forming apparatus including a developing device for developing a latent image electrostatically formed on an image carrier by causing a developer to deposit on a developing sleeve in the form of a magnet brush and contact the image carrier, the magnet brush uniformly rises in the form of a head in the axial direction of the developing sleeve and contacts the image carrier.